DC-DC converters that convert a variable DC input voltage to a DC voltage of a predetermined electric potential and outputs the converted DC voltage include step-up/down DC-DC converters that can step-up or step-down an input voltage and output the converted DC voltage. Inventions on such conventional step-up/down DC-DC converters are described in, for example, Patent Documents 1 and 2.
FIG. 6 illustrates the configuration of a step-up/down DC-DC converter disclosed in Patent Document 2. This step-up/down DC-DC converter includes nMOS transistors Q3 and Q4 connected in series to an input terminal receiving a DC voltage Vin supplied from a DC power source, such as a battery, and a grounding point; nMOS transistors Q1 and Q2 connected in series to an output terminal connected to a smoothing capacitor C2 and the grounding point; bleeder resisters R1 and R2 that divides an output voltage Vout to generate a feedback voltage FB; an error amplifier AMP1 that outputs a voltage VA corresponding to the potential difference between the feedback voltage FB and a reference voltage Vref; an inverting amplifier AMP2 that inverts an output VA from the error amplifier; a triangle-wave generator circuit TWG that generates triangle waves VTRI for pulse width modulation (PWM) control of the output voltage; and paired PWM comparators CMP1 and CMP2 that receive the generated triangle waves VTRI and the output VA of the error amplifier AMP1 or its inverted voltage VB.
An inductor (coil) L is connected to both a connection node N1 of the nMOS transistors Q1 and Q2 and a connection node N2 of the nMOS transistors Q3 and Q4, and the nMOS transistor Q2 is turned on/off by the output (PWM pulse) from the PWM comparator CMP1, and the Q4 is turned on/off by the output (PWM pulse) from the CMP2. The nMOS transistor Q1 is turned on/off by an inverting signal output from the PWM comparator CMP1, and the Q3 is turned on/off by an inverting signal output from the CMP2.
If an input voltage Vin is lower than a target output voltage, i.e., if a feedback voltage VA is higher than the peak voltage Vp of triangle waves VTRI, as illustrated in FIG. 7, the step-up/down DC-DC converter, which is illustrated in FIG. 6, drives the Q1 and Q2 with PWM pulses while the Q3 is being in a continuous ON state and the Q4 is being in a continuous OFF state to output a voltage Vout obtained by step-up of the Vin. If the input voltage Vin is higher than the target output voltage, i.e., if a voltage VB obtained by inverting the feedback voltage VA with reference to Vref is higher than the peak voltage Vp of the triangle waves, the Q3 and Q4 are driven by PWM pulses while the Q1 is being in a continuous ON state and the Q2 is being in a continuous OFF state to output a voltage Vout obtained by step-down of the Vin.